1. Field of the Invention
The present invention relates to a radiation-polymerizable composition, especially useful as or in a coating or ink, containing a radiation-curable acrylate resin oligomer component and a copolymerizable rheology modifier component. More particularly, the present invention relates to a printing ink which incorporates such a radiation-polymerizable composition.
2. Background of the Art
Printing inks generally are composed of coloring matter such as pigment or dye dispersed or dissolved in a vehicle. The ink can be a fluid or paste that can be printed onto a substrate such as paper, plastic metal, or ceramic and then dried.
Inks can be classified according to the substrate onto which the ink is intended to be applied or the method of application. For example, inks can be applied by raised type (e.g. letter press, flexographic), from a planar surface (lithographic), from a recessed surface (intaglio) or through a stencil (silk screen). Different methods of application and different substrates require different properties in the ink.
Rheology is one of the most important properties of the ink which must be suited to the substrate and manner of application. Printing inks are generally non-Newtonian fluids, which means that the rate of flow of the ink fluid is not proportional to the stress applied. Thus, some inks are required to be thixotropic, which means that the viscosity of the ink decreases with increasing agitation. Other inks are dilatant fluids in which increased agitation or rate of shear increases the viscosity and makes the ink less fluid.
The three main technologies being practiced today which make the bulk of the paints, coatings, inks and adhesive industries are solvent borne, water borne and zero volatile organic compounds (VOC). The main film forming process is either drying (evaporation of a solvent from polymer solution) or curing (two or more components reacting to form a thermosetting polymer). While the water borne systems are more environmentally friendly and acceptable from a waste and pollution standpoint, both solvent and water based systems are energy intensive, requiring drying ovens to remove the solvent or water. For several years there has been a technological push to eliminate solvents and water, i.e., to develop waterless zero VOC systems. Energy curing is one technology which has been investigated with this objective in mind. In an energy curable system, a relatively fluid formulation is instantly converted to a cross-linked polymer when exposed to energy from a visible or ultra-violet (UV) light source or an electron beam (EB). This technology reduces waste and requires less overall energy consumption. In many cases it vastly improves production speeds and produces properties such as high gloss and excellent abrasion resistance. Hence, energy curing is the technology of choice for many applications such as coatings for wood furniture, floor tiles, magazine covers, CD labels and jackets, high gloss optical fibers, electronic encapsulants and stereolithography. UV or EB curing can be accomplished by free radical, cationic, anionic, or charge transfer mechanisms.
One feature of importance to ink, as well as any other coating material, is adhesion. Inks adhere well to porous materials such as paper or cloth because, to some extent, the ink penetrates the fibers of the substrate. However, adhesion can be a problem with non-porous substrates such as plastic or metal.
Rheology modifying (RM) additives are often used in controlling the fluid flow characteristics of lithographic, letterpress and screen inks. In a press, UV cured inks experience high shear, their viscosity is reduced and they lose their optimum consistency. Inorganic additives such as silica have been used to control viscosity but they present problems such as reducing gloss in the final product and increase in viscosity over time.